AP EXPLAINS Trump’s push for border wall is not a new idea.

ALBUQUERQUE, N.M. (AP) — Donald Trump’s push for a border wall is not a new idea. The U.S.-Mexico border is already lined with intermittent miles of barriers. In some places, a tall fence ascends desert hills. In others, sturdy wire mesh or metal pillars end suddenly.

The pieces come from different moments in history when the U.S. government wanted stronger barriers to halt unwanted immigration, drug trafficking, Prohibition-era bootleggers and even meandering cattle.

When Trump formally accepts the GOP nomination for president on Thursday, the billionaire will likely repeat his promise to build a single wall to stem illegal immigration and drug smuggling.

Historians say that idea has been pursued for a century with spotty results due to changing politics and technologies and pressure to divert enforcement attention elsewhere.

Here’s a look at the history of U.S.-Mexico border barriers.

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BIRTH OF A WALL

The U.S. began constructing pieces of a wall between U.S. territory and Mexico during World War I, according to Kelly Lytle Hernandez, a University of California, Los Angeles history professor and author of “Migra!: A History of the U.S. Border Patrol.”

Ironically, there were no restrictions on Mexican immigration at the time because U.S. growers wanted a steady stream of agricultural workers. Mounted watchmen who patrolled from El Paso, Texas, to California were largely on the lookout for Chinese immigrants trying to illegally enter the U.S.

Pressure to build a wall came and went after Congress created the U.S. Border Patrol in 1924. Some fences and tin walls that had been built were abandoned or neglected until voters demanded new barriers. Meanwhile, many illegal crossers dug tunnels to dodge the walls and keep out of sight.

Entering the U.S. became more restricted as the U.S. struggled to recover from the Great Depression. A decade later, Organ Pipe National Monument in Arizona and other national parks along the border called for fencing to keep Mexican cattle and other livestock from overgrazing in protected areas.

The push for a wall got new life under President Richard Nixon with Operation Intercept — an effort to halt the flow of illegal drugs from Mexico. Later, when the North American Free Trade Agreement hurt the agricultural economy in Mexico in the 1990s, millions of migrants came to the U.S. illegally, prompting an expansion of border fences in urban areas such as San Diego and El Paso. That forced many migrants to try dangerous desert crossings into Arizona.

After the Sept. 11 terror attack, President George W. Bush signed the Secure Fence Act of 2006 authorizing the construction of 850 miles of border fencing. It also called for more vehicle barriers, checkpoints and lighting to help the Border Patrol spot crossers.

In 2011, President Barack Obama declared fencing along the U.S.-Mexico border was “now basically complete” — a claim rejected by many critics.

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THE WALL TODAY

Current barriers from the Pacific Ocean to the Rio Grande range from vehicle checkpoints made of steel beams to tin walls that can be easily climbed.

In large cities such as El Paso, motorists can drive next to a barrier. Some stretches feature murals honoring female migrants who have disappeared or small shrines dedicated to La Virgen de Guadalupe, the patron saint of Mexico and Mexican Americans.

The walls have been the scene of cross-border volleyball games and bilingual poetry competitions known as slams. Priests hold Mass and give Communion through the walls. Tourists sometimes go off-road to visit spots where barriers end to take selfies on both sides of the border.

Meanwhile, the mounted watchmen of a century ago have been replaced by border patrol vehicles and infrared cameras.

Hernandez said building a continuous wall would be a massive undertaking with uncertain results.

“There are always planes,” she said. “There will always be other ways to get across.”

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Follow Russell Contreras on Twitter at http://twitter.com/russcontreras . His work can be found at http://bigstory.ap.org/content/russell-contreras .